1. Field of the Invention
This invention relates to the preparation of adhesives, matrix resins for composites, molding powders, films, and laminates, especially those showing enhanced flow with retention of mechanical and adhesive properties.
2. Description of the Related Art
High performance polyimide adhesives are used in the aerospace industry, for example, in joining metals to metals or metals to composite structures. In addition they are rapidly finding new uses as matrix resins for composites, molding powders and films. These materials display a number of performance characteristics such as high temperature and solvent resistance, improved flow for better wetting and bonding, high modulus, chemical and hot water resistance, etc. One area of application is in the manufacture of lighter and stronger aircraft and spacecraft structures.
LARC-TPI is a thermoplastic polyimide well known as a high performance material (V. L. Bell, B. L. Stump and H. Gager, J. Polym, Sci., Poly. Chem. Ed. 14,2275 (1976); D. J. Progar, V. L. Bell and T. L. St. Clair, NASA Langley Research Center, "Polyimide Adhesives", U.S. Pat. No. 4,065,345 (1977); V. L. Bell, NASA Langley Research Center, "Process for Preparing Thermoplastic Aromatic Polyimides", U.S. Pat. No. 4,094,862 (1978); A. K. St. Clair and T. L. St. Clair, NASA Langley Research Center, "High Temperature Polyimide Film Laminates and Process for Preparation Thereof", U.S. Pat. No. 4,543,295 (1985); A. K. St. Clair and T. L. St. Clair, "A Multi-Purpose Thermoplastic Polyimide", SAMPE Quarterly, October 1981, pp. 20-25).
Its structure, as shown below, is derived from 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3'-diaminobenzophenone (3,3'-DABP) ##STR1## and is prepared in 2-methoxyethyl ether (diglyme). It was invented at NASA Langley Research Center and is a commercially available product sold by Mitsui Toatsu (Mitsui Toatsu Chemicals, Inc., New York) and the Rogers Corporation (D. C. Sherman, C. Y. Chen and J. L. Cercena, SAMPE Preprints, 33, 134, 1988) as a molding powder, polyamic acid solution and film.
The commercialization of the 3,3'-DABP component of LARC-TPI has not occurred in the United States because it has been shown to be a mutagen. (W. D. Ross, J. E. Nobel, J. A. Gridley, J. M. Fullenkamp, M. T. Winiger and J. A. Graham, "Mutagenic Screening of Diamine Monomers", NASA Contractor Report 166085, Monsanto Research Corporation, February 1983.) Hence, only experimental samples of this chemical can be purchased for research purposes. One U.S. company prepares and polymerizes the 3,3'-DABP in situ, limiting the exposure of workers to this diamine. However, this process is costly.
In addition, the original version of LARC-TPI, a solution of polyamic acid in diglyme, appears to have an undesirably large and/or broad molecular weight distribution, containing amine and anhydride functional end groups such that the initial imidized powders do not mold well and exhibit poor melt stability.
Bell (U.S. Pat. No. 4,094,862) discloses a polyimide of the same structural form as the one claimed by the present invention. Bell did not actually make a polyimide of this structure, and Bell does not mention any improvement over other polyimide structures that will result from the structure of the present invention. The oxygen flexibilizing groups of the present invention give the new polyimide good flow properties. Bell also used BTDA and 3,3'-DABP as possible precursors to his new polyimides, and makes no mention of the use of 3,4'-oxydianiline (3,4'-ODA) or 4,4'-oxydiphthalic anhydride (4,4'-ODPA). One purpose of the present invention, however, is to make an improvement on polyimides formed from BTDA and 3,3'-DABP.
Berdahl et al (U.S. Pat. No. 4,808,731) disclose that ODPA is a well known monomer used in the synthesis of polyimides having good thermal properties and high solvent resistance. Berdahl et al also use BTDA to create this type of polyimide. Berdahl et al do not mention the good flow properties of ODPA and 3,4-ODA caused by the presence of oxygen flexibilizing groups which make the present polyimide unique.
St. Clair et al (U.S. Pat. No. 4,389,504) and St. Clair et al (U.S. Pat. No. 4,497,935) both disclose a rubber toughened addition-type polymer composition made by chemically reacting an amine terminated elastomer and an aromatic diamine with an aromatic dianhydride with which a reactive chain stopper anhydride has been mixed, and utilizing solvent or a mixture of solvents for this reaction. The processes use ODPA and isomers of oxydianiline (although not 3,4'-ODA by name). The present invention, however, is producing a very different substance. No elastomer additions are made in the present invention, which, therefore, has very different characteristics, such as flow, etc.
St. Clair et al (U.S. Pat. No. 4,595,548) (hereinafter St. Clair et al '548) disclose a process for preparing essentially colorless polyimide film containing phenoxylinked diamines. One way to obtain this film is to use a polyimide made from ODPA and o,p'-ODA or m,m'-ODA. The use of 3,4'-ODA is not mentioned. St. Clair et al '548 is focused mainly on producing a clear film, and to achieve this result an essential part of the process is to use recrystallized or sublimated aromatic diamines. This is not the case with the present invention. There is no discussion on attempting to achieve tougher, higher molecular weights, or good adhesive qualities.
Olivier (U.S. Pat. No. 3,324,181) is concerned with endcapping polyimides to improve processing ability. It and the present invention both use phthalic anhydride as the endcapper. However, none of the polyimides that Olivier discusses has the formula of the polyimide of the present invention. The present invention can also use more mole percentage of phthalic anhydride as an endcapper than Olivier discloses, and somewhat different percentages of diamine and dianhydride. This suggests that simply following Olivier's disclosures would not lead to a processable polyimide.